(1) Field of the Invention
This invention relates to an improved method of activating carbonaceous material to maximize the product yield and prevent sintering on the gas distributor plate. More specifically, this invention relates to contacting carbonaceous material, preferably coal, in a plurality of fluidized beds at elevated temperatures whereby at least 10% of the thermal energy of activation is supplied by excess air.
(2) Description of the Prior Art
Sources from which activated carbon is derived include animal materials, such as bone and vegetable matter, such as wood and coconut shells. Activated carbon is also produced from coal. If coal is to be activated in a thermal process to produce granular carbon, the coal is exposed to an oxidizing gas, such as steam, air or carbon dioxide. The oxidizing gases react with the coal and cause an increase in the pore volume and surface area. The desirable properties of activated carbon stem from the increase in pore volume and surface area.
The processing steps for coal taken prior to activation of the carbon can be referred to as pretreatment or preconditioning. U.S. Pat. No. 3,843,559 by Repik et al. describes such pretreatment in detail. This patent is incorporated by reference.
To produce granular activated carbon from coal, three methods have been used. The first and most widely used process involves activation using a multihearth furnace. U.S. Pat. No. 3,539,467 to Bozarth et al. describes such activation. An alternative method of activation is a rotary kiln as described in Activated Carbon by J. W. Hassler, published by Chemical Publishing, Inc., New York, 1963. The third method of activation involves fluid bed technology, and one such two-step process has been described in U.S. Pat. No. 3,840,476 to W. J. Metrailer.
Briefly, in fluid bed activation, the pretreated feed material is introduced into an enclosed chamber. The chamber is provided with a gas permeable base plate called a distributor plate, through which fluidizing gases are admitted to the chamber from below to contact the bed of carbonaceous material and impart continuous movement to the particles comprising the bed. In this state of rapid continuous movement, the particles are fluid in nature and possess flow properties. Typical fluidizing gases include; (1) a mixture comprising nitrogen, carbon dioxide and steam from the combustion of natural gas with air, (2) a mixture comprised of combustion gases with addition of excess steam and (3) pure steam. At temperatures above 1,000.degree. F., the oxidizing agents within the fluidizing gases can react with carbon to produce an activated carbon product.
Unfortunately, because of various problems, fluid bed technology has heretofore not proved to be an entirely suitable commercial alternative to carbon activation using a multihearth furnace. A serious problem has been ash sintering which results in the formation of ash within the bed and on the plate of a fluid bed activator. Accumulation of sintered ash agglomerates, particularly on the gas distribution plate, could, of course, result in non-uniform gas distribution, lower production rates as the fluidized bed volume decreases from an increasing agglomerate volume, and ultimate shutdown of the equipment for cleaning and maintenance.
Some fluid bed activation methods have circumvented or minimized the potential for ash sintering by controlling both the temperature of the fluidizing gases and of the fluidized bed at about 1,400.degree. F.-1,800.degree. F., well below the sintering range. One such fluid bed activation process was reported by R. Bailey and J. Wilson in "A High Temperature Fluidized Process for the Activation of Anthracite" published in Journal of Brimingham University Chemical Engineering Society, 1974. This process is operated in a batch manner, with respect to solids flow, to activate anthracite coal in a single fluidized stage at bed temperature between 1,560.degree. F. and 1,780.degree. F. Steam is supplied at temperatures up to 1,670.degree. F. to serve as the fluidizing gas with gas distribution achieved using a perforated cone arrangement. This steam is also the reactant gas, and heat for the endothermic carbon-steam reaction is supplied by gas burners located in the reactor wall above the bed and discharging their hot combustion products into the bed. It is pointed out that the burners are designed and operated to insure that little free oxygen enters the bed.
In Metrailer, U.S. Pat. No. 3,840,476, the problems of the prior art were handled by development of a two-step fluid bed process wherein coke was first partially activated at low temperatures of from 500.degree. F. to 800.degree. F. followed by further activation at higher temperatures.
A second problem encountered in the development of fluid bed activation of carbon has been backmixing. Backmixing, a characteristic of single-stage fluidized beds, is a term used to indicate that the coal particles do not all remain in the fluid bed for the same period of time. Relatively low fluidizing gas and bed temperatures are utilized in a fluidized bed technique for activation of carbonized material for which J. R. Friday was granted U.S. Pat. No. 3,565,827 in 1971. This patent primarily discloses a means for minimization of particle backmixing which is indicated to be detrimental to product quality and yield.
A batch process for activation of carbonaceous materials using pure steam as the fluidizing gas is disclosed in U.S. Pat. No. 3,677,727 to A. Godel. Godel involves operation with zero fuel requirements with the heat requirements supplied by combustion of the activating off-gases by using at least two reactors which operate in an activating-reheating cyclic manner. While batch activation is occurring in a first reactor, the off-gases are being combusted in a second reactor where the heat is stored.
Relatively high fluidizing gas temperatures are indicated in U.S. Pat. No. 3,852,216 to Ninomiya and Kunii which discloses a fluidized bed process for producing coarse particles of activated carbon. The coarse particles are mixed in the fluidized bed with finely powdered inert material to provide a decrease in the required fluidizing velocity.
Other representative examples of the prior art patents relative to fluid bed and carbon activation or regeneration technology include U.S. Pat. Nos. 3,804,581; 3,756,922; 3,770,369; 3,617,727; 3,565,821; 3,153,633; 2,933,454; 2,851,428; 1,858,745; 1,843,616; British Pat. Nos. 546,531 and 1,302,456; French Pat. Nos. 942,699 and 951,153; and German Pat. No. 506,544.